Various methods for producing particles carrying radioactive nuclides are known. One method, disclosed in U.S. Pat. No. 3,334,050, comprises the application of high temperatures for sealing nuclides into the interstices of ion exchange cores by carbonizing the core.
This method has certain liabilities in that it is difficult to obtain a high yield of uniform and desired size cores because of the difficulty in controlling shrinkage of the particles. In addition, certain nuclides such as .sup.203 Mercury or .sup.125 Iodine are extremely volatile at temperatures used for carbonization and thus losses of these nuclides would be expected to occur. Furthermore, it has been found in practice that particles produced in this manner when utilized as an injectable preparation in animal research tend to agglomerate both in an injectable preparation and in vivo thus comprising test results.
Another technique which is set forth in U.S. Pat. No. 3,492,147 relys upon use of a non-reactive or inert substrate (e.g., sand, glass, etc.) to which a monomeric coating containing radioactive nuclides is applied and is polymerized by extraction of a catalyst from an acid bath which is contacted with monomer coated particles. It has been found in practice that with this process substantial undesired bulk polymerization occurs, which limits the usefulness of the product.
A further process of the prior art involves the incorporation of .sup.51 Cr acetylacetonate (a chelating agent) into polystyrene and polystyrene vinyl latices in toluene (non ion exchange resin) by a process called emulsion polymerization. This process tends to produce particles of very small dimensions (about 0.1 to 1.5 microns) which are too small for convenient use in animal circulatory studies.
In view of the foregoing, a new and improved product and method was needed for providing a tracer particle having an ion exchange resin core with a controlled thickness polymer coating. In particular the process of this invention has significant advantages over the prior art in that a uniform coating may be obtained in a short period of time (less than 3 hours) merely using a vessel containing the monomer and the cores having catalyst incorporated thereon. The ion exchange particles lend themselves ideally for incorporation of a large variety of different types of nuclides and in addition also provide advantage in that they are capable of being readily conditioned with catalyst (H.sup.+ or OH.sup.- depending on the monomer used) to effectuate the formation of a substantially non-leaching controllable thickness coating on the surface of the cores. As used herein the term leaching refers to the leaching of ions from the ion exchanger resin core through the coating. Applicants on the other hand have found that an inert particle such as sand does not have these properties and applicants were not able to produce a satisfactory coating using the same process as performed by them with the ion exchange resin.
The product of this invention has also unexpectedly been found to be non agglomerating in an injectable suspension, and when used in vivo or when stored in dry form.